Process vessels operating at elevated temperatures and containing corrosive fluids are used in several industries, including pressure reaction and letdown vessels such as autoclaves, heater vessels and flash vessels in hydrometallurgical processes extracting metal values from ores, concentrates and slags.
These vessels typically employ a structural metal shell, a refractory lining and a corrosion-resistant membrane lining between the structural shell and refractory lining. The membrane protects the structural shell from corrosion by process fluids such as high temperature acid, and the refractory lining protects the membrane from the elevated temperatures inside the vessel. The design of the refractory lining is intended to limit the temperature of the corrosion-resistant membrane to a target temperature, for example to protect the membrane by limiting its temperature to its maximum allowable service temperature. In a typical example, the membrane may be lead and the refractory lining may be insulating brick. A typical membrane may have a maximum allowable service temperature on the order of about 80° C. to 120° C.
This lining system works well for the body of the process vessel, but at the nozzles which connect the vessel to the process piping there can be insufficient space to include a sufficiently insulating thickness of the refractory lining inside the nozzle, which can result in the membrane being subjected to unacceptably high temperatures. If it is discovered that membrane temperatures are unacceptably high, then additional insulation must be retrofit into the nozzle.
Another problem in insulating the corrosion-resistant membrane from elevated process temperatures is that the heat can travel around the refractory lining, by conduction through the cover and nozzle flange, and so to the membrane via the steel shell.
Metallic sleeves and inserts have been used in the past, made of a wide variety of materials, including stainless steel, duplex and super-duplex stainless steels and titanium. These materials, while resistant to corrosive attack under various conditions, are deficient in thermal insulating properties.
There have also been instances where a polymeric resin, typically furan resin, has been poured between a metallic sleeve and the refractory lining of a nozzle to provide insulating value. However, these polymers have a limited service temperature above which they degrade, and it has been found that these insulating layers adhere the sleeve to the underlying refractory layer, making replacement of the sleeve difficult.
Accordingly, the need exists for more effective systems for insulating nozzles of process vessels containing corrosive fluids operating at elevated temperatures. In particular, the developing field of extracting metal values via hydrometallurgical processes requires, for some ore bodies, higher process temperatures than have previously been used. In the past, process temperatures of about 150° C. were typical. In order to process a wider range of ores, it is not uncommon for some currently used processes to operate at considerably higher temperatures and pressures, for example with the operating temperature inside the vessel being up to 300° C. Improved insulation methods are therefore required in order to maintain the membrane temperatures below an upper limiting temperature (such as the maximum allowable service temperature), by insulating it against higher process temperatures.